1. Field of the Invention
The present invention relates to a power supply. More particularly, the present invention relates to the control circuit of a switching power supply.
2. Description of Related Art
Power supplies are used for converting an unregulated power into a regulated voltage or current. FIG. 1 illustrates a conventional power supply. A control circuit 10 generates a switching signal VSW for controlling a transistor 20 to switch a transformer 30. A resistor 40 senses a switching current IP of the transformer 30 to control the switching. A resistor 45 determines the switching frequency of the control circuit 10. A terminal FB of the control circuit 10 is connected to an output of a feedback circuit 50. The feedback circuit 50 is coupled to an output terminal of the power supply to generate a feedback signal VFB. The duty cycle of the switching signal VSW is modulated in response to the feedback signal VFB to determine the power transferred from an input terminal of the power supply to the output terminal of the power supply.
Even though the switching technology reduces the size of power supplies, the electric and magnetic interference (EMI) generated by a switching device has an impact on the power supply and the peripheral equipments thereof. Therefore, apparatuses for reducing or preventing EMI (e.g. EMI filter, transformer protector, etc) are disposed in power supplies. However, such kinds of apparatus increase power consumption, the cost and the size of power supplies. Recently, frequency modulation or frequency hopping technologies are applied in many conventional technologies to reduce EMI. For example, the conventional technologies “Reduction of Power Supply EMI Emission by Switching Frequency Modulation” (IEEE Transactions on Power Electronics, VOL. 9. No. 1. January 1994) and “Effects of Switching Frequency Modulation on EMI Performance of a Converter Using Spread Spectrum Approach” (Applied Power Electronics Conference and Exposition, 2002, 17-Annual, IEEE, Volume 1, 10-14, March, 2002, Pages: 93-99) etc, and U.S. Pat. No. 6,229,366 “Offline Converter with Integrated Softstart and Frequency Jitter” (May 8, 2001) and U.S. Pat. No. 6,249,876 “Frequency Jittering Control for Varying the Switching Frequency of a Power Supply” (Jun. 19, 2001) etc., have been disclosed.
However, a disadvantage of the conventional technologies is that the output of the power supply will carry an unexpected ripple signal when there is frequency hopping. How the unexpected ripple signal is generated in the presence of frequency hopping will be described below with reference to the formulas.
An output power PO of the power supply is the product of an output voltage VO and an output current IO of the power supply, the equation of which is expressed as:PO=VO×IO=η×PIN  (1)
The relation between the input power PIN of the transformer 30 and the switching current IP can be expressed as:
            P      IN        =                  1                  2          ×          T                    ×              L        P            ×              I        P        2                        I      P        =                            V          IN                          L          P                    ×              T        ON            
Where η is the efficiency of the transformer 30, VIN represents an input voltage of the transformer 30, LP represents a primary inductance of the transformer 30, T represents the switching period of the switching signal VSW, and TON represents the on-time of the switching signal VSW.
Thus, equation (1) can be given by:
                              P          O                =                  η          ×                                                    V                IN                2                            ×                              T                ON                2                                                    2              ×                              L                P                            ×              T                                                          (        2        )            
It can be understood from equation (2) that the switching period T changes in response to the frequency hopping. When the switching period T changes, the output power PO changes accordingly. Therefore, the unexpected ripple signal is generated when the output power PO changes.
Another disadvantage of the conventional technologies is the unexpected range of frequency hopping. Since the range of frequency hopping is related to the setting of the switching frequency, the effect of reducing the EMI is limited in response to different switching frequency setting under different application needs.